7. Conclusion

On the other hand, average service time increases, while the line moves to the

As Figure 11 shows, a reduction in TP translates into a slight rise in average service time, making the average service time line (dotted red line) far away from PDF's head. As mentioned before, this causes a drop in the reliability. On the other hand, a fall in TP leads to a slight growth of variance. Change in variance causes

As inferred from Figure 11, the transition variance from 2.326 to 6.999 makes the queue length and queue peak change from 6.35 <sup>10</sup><sup>8</sup> and 1 to 2.2451 and 23,

right of the diagram.

respectively.

Figure 15.

96

Queue average and peak against TP and N, while m0 = 3, mb = 8, m = 3, and n = 1.

fluctuation in the queue average and peak.

Research Trends and Challenges in Smart Grids

CPSs, developing rapidly and covering eclectic domains, constitute thriving solutions for Smart Grid, the next-generation power grid systems. In this paper, we proposed a novel analytical model based on Markov chain for the MAC sublayer of IEEE802.15.4 standard. This model can provide a precise QoS to applications in which data generation proves periodic, such as AMI in Smart Grid. This is achieved by supplying the model with a MAC-level buffer and the reconsideration of idle mode. The model can provide QoS by reducing the impact of traffic rate fluctuation and the variation of the number of nodes. We incorporated variable idle state lengths so as to makes our study more pragmatic, and then the overall performance in terms of the end-to-end delay and reliability was evaluated. In this paper, the end-to-end delay refers to the interval between when a packet is generated and when a packet service is accomplished, including the time when in the queue as well as transmission time. We observed that the delay distribution of IEEE802.15.4 depends mainly on the MAC parameters and the collision probability.

Furthermore, using the probability density function of transmission time, we designed an optimum network meeting our QoS requirements. We analyzed the impact of MAC parameters and packet generation rate on the shape of the PDFs. In order to make our view more general and feasible, both saturated and unsaturated traffic has been applied, and no limitation is imposed on the queue length.

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Besides Monte Carlo simulations, we performed a field test on the protocol by building a WSN with self-designed motes, validating our model. Future work includes investigating the performance of our analytical model with a downlink stream.
